If such devices are used for concrete formwork, they are known as formwork panels. They are reusable and serve for supporting the concrete unti it has set. They are used for the fabrication of masonry walls. In this case, two generally parallel formwork panels delimit the thickness of the later wall. Such formwork panels are also used, however, for ceiling boarding, the formwork of joists, the formwork of piers, etc. In service, they have to meet numerous demands which are contradictory. For example, they must be light. The reason for this is that, as individual formwork panels, they have to be hand-led if possible by a single man or if possible by two men. But, even if the formwork panels are to lifted by a crane, they should be light because in this case several formwork panels are joined together. The normally used formwork panels are heavy, since the formwork board consists of a thick sandwich board with wood as the principal constituent. The frame and the webs supporting the formwork board from behind are made of steel. The disadvantages of these formwork panels are as follows:
(a) Due to the high weight, the formwork panels are difficult to handle.
(b) Due to the high weight, only a certain number of them can be transported on trucks.
(c) The formwork panel must fit in the frame. Special production techniques have to be used here as only the frame is made of dead material, while the formwork board is made of live material.
(d) The peripheral lines of the formwork board stand out in many places on the finished concrete since the frame protrudes at least with one rib up to the concrete. With two formwork boards next to each other, there are thus three parallel, closely adjacent ribs protruding out of the finished wall.
(e) The formwork board absorbs water. As long as it is new, this is not very much. However, when it later separates into fibers, it absorbs more and more water. This means that the concrete has too little water during setting on the site and it then gets the familiar air voids.
(f) A loss of water can also take place in the narrow gaps between the edge of the formwork board and the frame. This is all the more so as the hydrostatic pressure on a, for example, 2.50 m high formwork panel with filled concrete is, after all, quite considerable.
(g) The formwork board determines by its surface quality the surface quality of the later concrete. The smoother it is, the smoother will also be the masonry wall or the ceiling or similar. Even with very high-grade formwork boards, the surface quality deteriorates over time due to separation into fibers. If the surface quality were very high, this would also have the advantage that a very thin layer of cement separates out directly next to the formwork board surface, which is desirable both for aesthetic reasons and for reasons of subsequent aftertreatment. In the case of the known devices, the formwork board is either very rough from the outset or it becomes very rough during use.
(h) When the concrete has been poured between formwork panels, it is, as is known, compacted by vibrators. In this operation, the concrete moves down very slightly. In the region of the surface of the formwork board, the concrete, of course, moves down all the more readily the smoother it is.
(i) The laitance is anything but a chemically neutral substance. Rather, it attacks metal. This means that the retaining edge of the frame legs of the formwork panel corrodes over time.
(j) For reason of equitable work distribution, building is also to take place as far as possible during the winter. During setting of the concrete, a small amount of heating occurs. Over wide temperature ranges, this is immaterial. As from a temperature of, for example, -10.degree. C., the formwork panels dissipate so much heat that the concrete no longer sets. The heat is dissipated in particular in the region of the formwork board mounts as they, after all, come into contact with the concrete directly. Wherever the, in themselves, high-grade formwork boards have been used in sandwich design, they have a poor heat insulation. They lean heavily against the crossmembers of the formwork panel frame, and these crossmembers then act practically as cooling ribs for the area behind them. Thus, it can happen that the metal parts of the formwork panels stand out on the concrete like a grid. This makes a structure either totally or partially worthless.
Merely for the sake of weight reduction, in recent years attention has turned to formwork panels made of aluminum. However, aluminum is very expensive and can only be welded by special weldings, is attacked even more by the laitance and is dented much earlier than the formwork panels of the structure mentioned above. In aluminum formwork, the formwork board is frequently also made of aluminum. On aluminum, however, the concrete begins to cake after only the second or third formwork application, so that demolding presents problems.